This invention relates to controlling charged particle beams and in particular to an ion optical system in a mass spectrometer. It also relates to an optical system as such for charged particles.
The present invention will be described mainly with reference to an Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) instrument having an Inductively Coupled Plasma ion source and a quadrupole mass analyser, however it is to be understood that it may be applied to other types of mass spectrometers employing other types of ion sources and other types of mass analyser. A charged particle optical system per se, according to the invention, may be used in charged particle beam applications, including particle-emission micro analysis, microscopy and thin film technology.
There are a number of applications where it is desirable to know the concentration of particular elements within a material. One way of carrying out such an analysis is to ionise a sample of the material of interest and then determine the relative abundance of particles with the mass to charge ratio representative of the element of interest. Such a determination may be carried out in a mass spectrometer. In an ICP-MS the sample is ionised by injecting it into an inductively coupled plasma and a jet of gas is abstracted from the plasma source and passed into a vacuum chamber. This jet of gas is a beam of particles consisting of a mixture of sample ions and uncharged particles. Before analysis in the mass spectrometer it is necessary to separate the ions from the neutral particles. This is because the neutral particles or gas interferes with the action of the mass analyser and leads to high background. Background may be defined as counts received even when the mass of interest is not present in the sample. Such background together with the instrument sensitivity determines the lower limit of concentration of an element in a sample unambiguously detectable by the system.
It is also necessary to focus the ion beam onto the input of the mass analyser. This is complicated by the fact that the ions do not all have the same energy. There is a variation in ion energy both between ions of different masses and between ions of the same mass.
Control of the ion beam is generally effected via shaped electric fields which in turn are created by suitably positioned electrodes operated at controlled voltages. This set of electrodes is normally referred to as an ion optics system. In existing ICP-MS systems the ions pass through the electric field structure and in the process their path is bent in a predetermined manner. A common approach is to use a series of electrodes arranged with cylindrical symmetry which create curved electric fields. As the ions pass through these fields their path is bent in such a way as to cause the beam to refocus to a desired point. This is exactly similar to the process for focussing light rays and these electrodes are commonly referred to as ion optics lens elements and the system is referred to as a transmissive ion optics system.
A transmissive ion optics system re-focuses the ion beam but it does not separate ions from neutral particles. The conventional way of achieving such separation uses an on-axis metal plate which physically blocks the neutral particles. This is variously called a photon stop, neutral stop or stopping plate. The ion beam is deflected to a donut shape around this stop by the electric fields and then re-focussed after the stop. Systems having such a stop structure have numerous disadvantages. Firstly the efficiency with which the beam is deflected around the neutral stop is usually relatively poor and very dependent on the mass of the ions. Light ions tend to be deflected too far and a large proportion are lost. Heavy ions are not deflected far enough and hit the plate also resulting in their loss. The overall ion collection efficiency is low and mass dependent. Further, for successful deflection and subsequent re-focussing the deflection angles must be kept reasonably small which means the ion optics becomes quite long. This in turn means the ion path through the ion optics is long with the net result that there is considerable loss of ions to collision with neutral particles. This further reduces the ion collection efficiency. Yet a further disadvantage is that such systems exhibit considerable change in image position with initial ion energy (often called chromatic aberration from analogy with light optics). Thus only one mass can be brought to an accurate focus at the mass analyser entrance resulting again in loss of sensitivity for ions of other masses. In theory this can be corrected by changing the electrode voltage depending on the mass of interest. In practice however, such dynamic lens elements exhibit very poor stability due to surface charge build up.
Several on-axis systems are currently offered for sale and differ from each other principally in the position of the neutral stop, the voltage (if any) applied to such a stop and the number and arrangement of the ion optics elements. For example the Sciex xe2x80x9cElan 6000xe2x80x9d places a grounded neutral stop in the throat of the skimmer cone (they call it a shadow stop) and use a single lens element of large diameter directly behind this shadow stop. By contrast, the Varian Ultramass uses 6 ion optics lens elements with the neutral stop in the middle of the ion optics elements and applies a voltage to the neutral stop.
In an attempt to overcome some of the problems of on-axis neutral stop systems, some designers use an approach of deflecting the ion beam out of the neutral beam. One example of such an arrangement is the Hewlett Packard omega lens system used in their 4500 ICP-MS. This system uses an arrangement of 6 lens elements followed by a four electrode deflector which cause a lateral shift in the wanted ion beam. The system eliminates the need for an on-axis neutral stop but in practice the system suffers from most of the same problems as the on-axis approach.
An object of the present invention is to reduce the above discussed limitations by providing an alternative to a transmissive ion optics system.
In addition to creating electric field patterns which deflect ions as they pass therethrough, it is possible to create fields which ions do not penetrate but will instead reflect off. By controlling the shape of the electric field it is possible to direct such reflection so as to create a focussing effect. In effect, an ion mirror instead of an ion lens is provided.
Thus according to the invention there is provided a mass spectrometer which includes a source for providing a beam of particles including ions, a mass analyser and an ion detector for receiving ions from the beam of particles for spectrometric analysis, and an ion optics system for reflecting ions from the beam to the mass analyser and detector.
Preferably the ion optics system comprises multiple electrodes and establishes an electrostatic field which simultaneously reflects and focusses the ion beam. The focussing may be onto the mass analyser entrance.
A particular type of ion mirror called a reflectron has been used in time of flight ICP-MS instruments but only to increase the apparent ion path length in the drift region employed in such instruments. This region is after the separation of ions from neutrals has been effected and such structures are not used as ion focussing devices. To the applicant""s knowledge, the significant advantages which may be achieved via use of an ion mirror in the ion/neutral separation stage and ion focussing stage have not been previously recognised.
In an ion mirror, the ions are reflected but the neutral particles, being uncharged, pass straight through the field. According to the invention, the electrodes and their support structures are designed so that they are out of the path of the neutral particles, that is, there are no physical obstructions in the path of the neutral flux and these particles will therefore pass through the entire ion reflective structure without being scattered. This may be achieved, for example, by arranging the electrodes in a ring through which neutral particles pass. Further, a pumping port may be positioned so as to intersect this flux, and the majority of the neutrals can be removed after a minimum one pass through the vacuum chamber and before they have a chance to scatter off the chamber walls. This leads to lower background pressure in the vacuum chamber. The ions can in principle be reflected through any included angle but reflection through a substantial angle of preferably 90xc2x0 or greater makes it easier to physically accommodate the mass discriminator and suitable pumping port within the available space. In this way separation of ions from neutrals can be achieved very efficiently.
A further advantage of a reflective ion optics system is that the ion path can be made very short. For example, in an embodiment of the invention the ion path from skimmer cone to mass analyser entrance is only 6 cm. By contrast, a typical conventional ion optics system has ion path lengths of about 17 cm and some commercial systems are still longer. Since the chamber is never free of neutral particles (even with a preferred pumping arrangement to be described below), collisions between ions and neutrals will occur and in each such collision the ion involved is lost. The longer the path length the more ions will be lost. Thus, by reducing the path length the number of lost ions is reduced. Alternatively, higher pressures can be tolerated in the chamber, for the same ion loss, which leads to smaller pumps and a cheaper system.
Another advantage of an ion mirror system is that it is easier to create complex spatial field patterns which can correct out aberrations caused by varying ion energies.
It is also possible to electrically steer the beam so that the focus point coincides with the entrance aperture to the mass analyser.
Yet a further advantage of a mirror based ion optics system is that it is possible to create a mirror field which is not infinitely strong such that only ions of interest are reflected off the field in the intended fashion. That is, the field can be adjusted so that ions with energy greater than that which can be handled by the mass analyser are able to penetrate the field and continue on with the neutral beam to the vacuum pump. In this way the mirror can also act as an energy filter removing high energy ions which would otherwise cause undesirable background in the mass analyser. Thus the invention also encompasses a method of operating a mass spectrometer of the invention for filtering higher energy ions from lower energy ions.
The invention encompasses mass spectrometers employing any type of ion source including an inductively coupled plasma (ICP) ion source as described above. Examples of alternative atmospheric pressure ionisation sources are an electrospray ion source or a chemical ionisation source.